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Chapter 12Stoichiometry

12.1 The Arithmetic of Equations

12.2 Chemical Calculations

12.3 Limiting Reagent and Percent Yield

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How do you figure out how much starting material you need to make a finished product?

CHEMISTRY & YOUCHEMISTRY & YOU

When making bikes, you need parts like wheels, handlebars, pedals, and frames.

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Using EquationsUsing Equations

Using Equations

How do chemists use balanced chemical equations?

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Chemists use balanced chemical equations as a basis to calculate how much reactant is needed or how much product will be formed in a reaction.

Balanced Chemical Equations

Using EquationsUsing Equations

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The calculations of quantities in chemical reactions is a subject of chemistry called stoichiometry.• For chemists, stoichiometry is a form of

bookkeeping.

• It allows chemists to tally the amounts of reactants and products using ratios of moles or representative particles.

Using EquationsUsing Equations

Balanced Chemical Equations

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Cayla is using a recipe to make chocolate chip cookies. She wants to double the number of cookies that the recipe will make. The original recipe calls for 2 cups of chocolate chips. How many cups of chips should Cayla use for a double recipe?

A. 2 cups C. 1 cup

B. 4 cups D. 8 cups

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Chemical EquationsChemical Equations

Chemical Equations

In terms of what quantities can you interpret a balanced chemical equation?

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Ammonia is produced industrially by the reaction of nitrogen with hydrogen.

N2(g) + 3H2(g) 2NH3(g)

• The balanced chemical equation tells you the relative amounts of reactants and product in the reaction.

Chemical EquationsChemical Equations

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Ammonia is produced industrially by the reaction of nitrogen with hydrogen.

N2(g) + 3H2(g) 2NH3(g)

• The balanced chemical equation tells you the relative amounts of reactants and product in the reaction.

• Your interpretation of the equation depends on how you quantify the reactants and products.

Chemical EquationsChemical Equations

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A balanced chemical equation can be interpreted in terms of different quantities, including numbers of atoms, molecules, or moles; mass; and volume.

Chemical EquationsChemical Equations

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Chemical EquationsChemical Equations

The table below summarizes the information derived from the balanced chemical equation for the formation of ammonia.

N2(g) + 2H2(g) 2NH3(g)

+

2 atoms N + 6 atoms H 2 atoms N and 6 atoms H

1 molecule N2 + 3 molecules H2 2 molecules NH3

10 molecules N2 + 30 molecules H2 20 molecules NH3

1 + 3 2

1 mol N2 + 3 mol H2 2 mol NH3

23 g N2 + 3 2 g H2 2 17 g NH3

34 g reactants 34 g products

Assume

STP

22.4 L N2

+

67.2 L H2 44.8 L NH3

6.02 1023

molecules N2( ) 6.02 1023

molecules H2( ) 6.02 1023

molecules NH2( )

22.4L

22.4L

22.4L

22.4L

22.4L

22.4L

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2H2S(g) + 3O2(g) 2SO2(g) + 2H2O(g)

Sample Problem 12.2Sample Problem 12.2

Interpreting a Balanced Chemical EquationHydrogen sulfide, which smells like rotten eggs, is found in volcanic gases. The balanced equation for the burning of hydrogen sulfide is:

Interpret this equation in terms of

a. numbers of representative particles and moles.

b. masses of reactants and produces.

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Mass and atoms are conserved in every chemical reaction.

Chemical EquationsChemical Equations

• Molecules, formula units, moles, and volumes are not necessarily conserved—although they may be.

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Interpret the following equation in terms of volumes of gas at STP.

2H2(g) + 2NO(g) N2(g) + 2H2O(g)

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Interpret the following equation in terms of volumes of gas at STP.

2H2(g) + 2NO(g) N2(g) + 2H2O(g)

44.8 L H2(g) + 44.8 L NO(g) 22.4 L N2(g) + 44.8 L H2O(g)

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Key ConceptsKey Concepts

A balanced chemical equation provides the same kind of quantitative information that a recipe does.

Chemists use balanced chemical equations as a basis to calculate how much reactant is needed or product is formed in a reaction.

A balanced chemical equation can be interpreted in terms of different quantities, including numbers of atoms, molecules, or moles; mass; and volume.

Mass and atoms are conserved in every chemical reaction.

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Glossary TermsGlossary Terms

stoichiometry: that portion of chemistry dealing with numerical relationships in chemical reactions; the calculation of quantities of substances involved in chemical equations

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The Mole and Quantifying Matter

BIG IDEABIG IDEA

• Balanced chemical equations are the basis for stoichiometric calculations.

• The coefficients of a balanced equation indicate the number of particles, mole, or volumes of gas in the reaction.

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END OF 12.1END OF 12.1